The present invention relates to a multi-layer-coated member constituted by an ultra-hard alloy substrate of high-speed steel, cemented carbides, cermets, etc. coated with a plurality of layers excellent in oxidation resistance and/or wear resistance, particularly those suitable for cutting tools such as drills, end mills, throwaway chips for milling machines, etc.
Many attempts have already been reported to provide ultra-hard alloy substrates such as high-speed steel, cemented carbides, cermets, etc. with ceramic coatings excellent in oxidation resistance and wear resistance, thereby achieving long life due to an effective combination of their properties. The coating layers of coated tools have widely been composed of TiN, TiCN, etc. excellent in wear resistance. However, metal nitrides such as TiN are easily oxidized at high temperatures, resulting in extreme deterioration of wear resistance.
To solve the problems of the oxidation of TiN coatings, proposal has recently be made to add Al to these coatings to improve their wear resistance, oxidation resistance, etc. See Japanese Patent Laid-Open No. 62-56565, and Japanese Patent Publication Nos. 4-53642 and 5-67705.
The coating methods of ultra-hard alloy substrates are generally classified to chemical vapor deposition (CVD) methods and physical vapor deposition (PVD) methods. It is known that coatings formed by the PVD methods such as an ion plating method, a sputtering method, etc., serve to improve the wear resistance of the substrates without deteriorating their mechanical strength. Accordingly, cutting tools such as drills, end mills, and throwaway chips for milling machines that require high mechanical strength and chipping resistance are coated by the PVD methods at present.
In the above Al-containing coating layers proposed by Japanese Patent Laid-Open No. 62-56565, for instance, coating layers composed of carbides, nitrides or carbonitrides of Ti and Al provide ultra-hard alloy substrates with higher oxidation resistance and wear resistance than those containing no Al. It is, however, pointed out that the Al-containing coating layers rather deteriorate the mechanical properties of the ultra-hard alloy substrates. While the inclusion of Al into the coatings leads to improvement in the chemical properties of the coating surfaces, it deteriorates the fracture toughness of the coatings. Particularly when coated ultra-hard alloys are used for high-speed cutting tools, their teeth are extremely heated, resulting in oxidation and rapid wearing of the coatings and deterioration of the coatings by thermal shock and galling, and thus a decrease in life.
Cutting speeds are recently increasing, and severe cutting conditions are required in many cases as in the cutting of heat-treated high-speed steel. To cope with such conditions, improvement is desired.
Also proposed is the formation of an outermost layer of TiAlON, etc. to improve the oxidation resistance of the coated members (Japanese Patent Laid-Open No. 7-328811). However, the mere formation of an outermost layer consisting of oxides of Ti and Al fails to provide enough oxidation resistance to withstand severe working conditions.
It is further proposed that alumina layers generally formed by CVD methods are formed as outermost layers by ion plating methods (Japanese Patent Laid-Open No. 9-192906). However, the alumina layers formed by the PVD methods do not have sufficient adhesion to the underlying layers, resulting in peeling of the alumina layers by impact in actual cutting operation.